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Intelligent lighting for the 21st century | Part 3

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Extended wavelength response

Traditional vision applications are limited to the recording and capture
of images in the visible and near infrared (NIR) spectrum. This limitation
is caused by silicon, the material used in the majority of sensors.

In
silicon, photons in the visible wavelength have enough energy to
release the electrons and penetrate material such as silicium. Shortwave
light on the other hand is richer in energy, but sensors with quartz glass
or removable cover glass are necessary for the shortwave light to reach
the photosensitive silicon layer.

Diagram of different materials

UV imaging

One method of extending a sensor's response towards
shorter wavelengths is to use a fluorescent coating. One type is known
as a 'lumogen coating', which fluoresces when struck by a UV photon.
This fluorescence is detected by the pixels in the adjacent light sensitive
layer, as the wavelengths emitted by the coating are close to the peak
responsivity of the sensor.

The lumogen coating process needs to be highly accurate as the
thickness of the coating is critical if good results are to be obtained. If
the coating is too thick, the fluorescent emission is scattered and
absorbed within the coating itself and if the coating is too thin, photons
will not be detected. In order to ensure excellent UV image quality, this
type of sensor must always be used with UV transmissive lenses fitted
with visible cut and UV pass filters.

The lumogen coating process

In physics infrared radiation describes electromagnetic waves in the
spectral area between visible light and longwave terahertz radiation.
Infared imaging is a method to convert radiant energy in the infrared
wavelengths into a detectable or measurable form. The spectral range
of 780 nm to 1 mm is called infrared.

The spectral range is divided into
near infrared (NIR, 750 nm - 1400 nm), short wavelength infrared (SWIR,
1.4 µm-3 µm), mid wavelength infrared (MWIR, 3 µm - 8 µm), long wavelength infrared (LWIR, 8 µm-15 µm) and far infrared (FIR, 15 µm - 1000 µm).
The human eye reacts to visible light in the wavelength of about 390 to
750 nm. The NIR-spectrum is adjacent to this visible spectrum.
Silicon based CCD and CMOS sensors are used where possible for NIR
applications due to their lower cost compared with other sensor materials.

As silicon based sensors only show a limited efficiency in
converting photons into electrons in the wavelength >1000 nm,
manufacturers try to use special circuits or sensor coatings to get the
remaining sensitivity beyond 1000 nm.

Diagram

SWIR or short wavelength infrared is a very interesting spectral range.
Images created by SWIR cameras are almost similar to those taken by a
monochrome CCD camera, but the detector material is sensitive in a
wavelength band where water vapour causes maximum absorption
and silicon is transparent. Infrared imaging can be used to detect
features that are not apparent in visible wavelengths for applications
such as inspection of silicon wafers or solar cells, sorting of fruits or
vegetables, control of plant growth, recognition of safety features, etc.

Shortwave infrared cameras can be cooled or not, depending on the
application requirements. Uncooled sensors are typically used for
imaging up to 1.7 µm, cooled sensors offer improved reduced noise on
higher speed sensors, but are generally a more expensive solution due
to the thermoelectric cooling.

Following the SWIR-range is the mid wavelength infrared (MWIR,
3 µm - 8 µm). Cameras for this range are often called thermal cameras
and are most commonly based on Mercury Cadmium Telluride (MCT) or
Indium Antimonide (InSb) sensors. Again using the photoelectric effect,
MCT based sensors offer sensitivity at around 3 - 8 µm, where InGaAs is
insensitive. As with shortwave infrared, midwave infrared imaging can
be used for detecting reflected or emitted infrared. This sensor type
requires cooling to ensure that the detected signal is not saturated by
inherent dark current in the camera.

Longwave infrared (LWIR): Short and mediumwave infrared imaging
sensors use the photoelectric effect to provide a signal and produce an
image. Longwave infrared detectors (or micro bolometers) detect heat
radiation by measuring changes in capacitance or resistance within the
structure of the pixels. Commonly based on Amorphous Silicon (ASi) or
Vanadium Oxide (VO), these sensors can detect wavelengths of 8 to
14 µm where the measured values relate to the temperature of an
object.

This enables LWIR cameras to work where there is no infrared
source and with objects at much lower temperatures compared to
short and midwave infrared. As this technology is not dependent on an
external IR source for imaging, it can be used for security and surveillance applications in all lighting conditions.

Longwave infrared

Existing longwave IR detectors are relatively low resolution with pixel
resolutions typically of 320 x 240 or 640 x 480, but next generation
sensors are now coming to market which offer megapixel resolutions.

Optics

SWIR and MWIR cameras use standard glass lenses as glass has good
transmittance at these wavelengths. For LWIR, an alternative lens type
is required with sapphire, crystalline silicon and germanium being the
most common materials used. These are expensive to source and
process and in most cases are of fixed focal lengths, although zoom
lenses are becoming more common.

Thermography - temperature measurement

Short, medium and longwave infrared can all be used for thermography.
LWIR cameras are most commonly used for this, but SWIR and MWIR
sensors can be used for materials at higher temperatures. For example
when heated to high temperatures, steel emits wavelengths in the visible
spectrum - the shorter the wavelength, the higher the temperature.
Thermography is a large subject with temperature measurements
dependent not just on emissivity of an object, but also on reflectivity
and transmission of IR by an object. The camera also needs to be
calibrated using a black body (a perfect thermal energy emitter) so that
sensor readings can be accurately related to specific temperatures,
down to a single degree Centigrade.

The images show a scene under visible light (top), SWIR (left) and LWIR
(right). The car on the right has been driven recently, as can be seen
from the hot tyres and bonnet in the LWIR image. The cars centre and
left have been stationary for some time, although the car on the left has
a warm bonnet having been heated by sunlight. The SWIR image shows
the glass as non-transparent, while the visible image shows none of
this information.

Scene under visible light

Scene under SWIR

Scene under LWIR

The images that follow show surface mounted LEDs both with and
without cooling. The images use pseudo-colour to indicate the change
in temperature and clearly demonstrate the value of this technique.
There are important security and medical uses for thermal imaging in
the detection of potentially harmful fever among groups of people and
these can be deployed at any transport hub. Other uses for thermal
imaging include carpool lane checking (detecting the number of
passengers in a vehicle) and various recycling applications.